The present invention relates to an apparatus constructed by a combination of a mixture separating means and a mass spectrometer, which apparatus is important for separation and analysis of human body relating mixture compounds such as saccharides, peptides, proteins, and so on. Particularly, it relates to a liquid chromatograph/mass spectrometer and a capillary electrophoresis/mass spectrometer.
At present, importance is attached to the development of a method for mass analysis of human body relating compounds in the field of analysis. Human body relating compounds are generally existing as mixture compounds in solution. The development of an apparatus constructed by a combination of a mixture separating means and a mass spectrometer has progressed. A liquid chromatograph/mass spectrometer is a typical example of the apparatus. A liquid chromatograph is superior in separation of mixture compounds but it cannot identify compounds. On the other hand, a mass spectrometer is superior in the ability of identifying compounds sensitively but it cannot analyze mixture compounds. Therefore, a liquid chromatograph/mass spectrometer using a mass spectrometer as a detector of a liquid chromatograph is very effective for analysis of mixture compounds.
FIG. 23 is a block diagram showing the overall structure of a conventional liquid chromatograph/mass spectrometer. A sample solution separated by a liquid chromatograph 1 is introduced through a connecting tube 2 into an ion source 3. The ion source 3 is controlled by an ion source power supply 4 through a signal line 5a. Ions concerning sample molecules, which are generated by the ion source 3, are introduced to a mass analysis region 6 and mass analyzed. The mass analysis region 6 is evacuated to a vacuum by a vacuum system 7. The ions thus mass analyzed are detected by an ion detector 8. A detection signal is fed through a signal line 5b to a data processing unit 9.
As described above, the principle of the liquid chromatograph/mass spectrometer is simple but there arises a problem that the liquid chromatograph using samples in solution is incompatible with the mass spectrometer using ions in a vacuum. Accordingly, the development of an apparatus constructed by a combination of a liquid chromatograph and a mass spectrometer and the development of a method therefor are attended with great difficulties. There have been developed some methods to solve this problem. Of the methods, the likeliest is a spray ionization method in which an effluent from a liquid chromatograph is nebulized so that sample molecules contained in generated droplets are ionized and introduced into a mass analysis region.
As an example of the spray ionization method, an electrospray method described in Analytical Chemistry, 59, 2642 (1987) will be described below. FIG. 24 is a sectional view showing the structure of a liquid chromatograph/mass spectrometer equipped with an electrospray ion source. A sample solution eluted from a liquid chromatograph 1 is introduced through a connecting tube 2 and a connector 10 into a capillary 11 for nebulization. By application of a voltage of the order of kV between the nebulization capillary 11 and a counter electrode 12, small charged droplets of the sample solution are conically nebulized from an end of the nebulization capillary, that is, a so-called electrospray phenomenon occurs. In the electrospray method, an output 13 for nebulizing gas is provided so that gas such as nitrogen gas is poured from the surroundings of the nebulization capillary 11 to thereby accelerate the vaporization of the small charged droplets. Further, the gas such as nitrogen gas is blown toward the generated small charged droplets from an outlet 14 for vaporizing gas provided in the counter electrode 12 side to thereby accelerate the vaporization of the small charged droplets. Ions thus generated are introduced through an ion sampling aperture 15 into a vacuum 6 and mass analyzed by a mass analysis region 6 under a high vacuum.
On the other hand, a structure shown in FIG. 25 was conventionally used as an ion detector to improve the signal-to-noise ratio (SIN) in the mass spectrometer. An ion deflecting electrode 16 is provided in the rear portion of a mass analysis region 6 for mass separation under a high-frequency electric field to deflect mass-separated ions. The deflected ions are accelerated at a voltage of the order of kV and collide with a dynode 17 to produce secondary electrons. Secondary electrons are emitted from the secondary electron-producing dynode 17 with which the ions collide. The emitted secondary electrons are detected by an electron detector 18 such as an electron multiplier. By the structure shown in FIG. 25, neutral molecules having no charge, charged droplets or droplets having no charge are prevented from being detected as a signal by the ion detector 8, so that improvement in S/N is attained to some degree.